This invention relates in general to a heating apparatus and method and, more particularly, to a fusing apparatus and method for fusing toner to a copy sheet, such as paper.
In a typical photocopy apparatus, an image is projected onto the surface of a charged photoconductor. The conductor may be a selenium drum or, an organic photoconductor that is deposited on the surface of a flexible belt. The photoconductor is initially electrostatically charged. The charged photoconductor is then selectively discharged in regions exposed to the image. The remaining charged regions form a latent electrostatic image. The photoconductor passes near a supply of toner particles which are attracted to the charged, latent image. The photoconductor is then brought into proximate location with a copy sheet. The toner particles are transferred to the copy sheet by suitable transfer apparatus, typically a transfer corona, transfer roller or other alternative device. The copy sheet then passes through a thermal fusing station. The thermal fuser uses heat to partially melt and fix the toner particulars to the copy sheet. The fusing station may also use a pressure roller to assist the fusing process. A similar fusing operation occurs in laser print engines where a photoconductive member is selectively discharged by a laser to create a latent image. Such laser print engines are found in laser printers and laser facsimile machines.
Conventional fusing stations suffer from a number of drawbacks. They have a thermal fusing element such as a roller or one or more strip heaters. In both cases, the temperature of the fusing element must be held to a relatively close tolerance for successful fusing. Conventional fusing stations rely upon one or more thermistors to sense the temperature at one or more locations of the fusing elements. Suitable control and power circuitry responds to thermistor signals to adjust the temperature of the fusing element. Such control systems are expensive, complex, and unreliable. Moreover, the results of such operations, especially at high speeds, is disappointing. Indeed, only the more expensive high speed printers and copiers can afford the required overhead of components and controls in order to quickly produce numerous copies with acceptable fusing. Accordingly, there exists a long felt need in the art for a simpler, self-regulating fusing station that has fewer components and permits the user to run the fusing apparatus at a relatively high speed. This invention meets those needs.
This invention uses a heating element that is manufactured in accordance with the method of manufacture disclosed in U.S. Pat. No. 5,039,840, the entire disclosure of which is herein incorporated by reference. The referenced patent describes how a conductive material is flame spray deposited on a surface to create a heating element. During flame spraying, the metallic powders are partially oxidized to produce a resistive coating comprising a conductive oxide layer with controlled resistance. The patent applies the heating elements to electric kettles and other domestic appliances. This invention adapts the conductive oxide layers to a fusing apparatus. In particular, this invention provides a self-regulating fusing apparatus using conductive oxides. The apparatus does not require sensors or feedback networks. Instead, the conductive oxide self regulates and draws the necessary current to the heat portions of the fusing element.